1. Field of the Invention
The present invention relates to a light emitting element on which a light emitting diode (hereinafter referred to as LED) is loaded and a light emitting device using the light emitting element, more specifically, to a surface-mounted LED improved to increase heat release efficiency and light emitting efficiency and also to a light emitting device using it.
2. Description of Related Art
Conventionally, an LED, which is a semi-conductor formed by a chemical compound is widely utilized as a light emitting element, with a characteristic of long operating life and miniaturization. Because an LED of semi-conductor of gallium-nitride system compound for emitting blue color is developed and manufactured, a field of application of the LED is extended to a color display, and the application field is increasingly extended to a small color back light for a mobile phone, a vehicle-mounted display, further, a light emitting device of high brightness and high output for illumination, and further high brightness and long operating life are required for the LED.
Recently, in particular, a great number of light emitting elements of surface-mounted types, using the LED are manufactured because of characteristics of miniaturization and well-suited to mass production. However, when these light emitting elements are used for high brightness and high output, measures for heat release problem. That is to say, because driving current and brightness are approximately in a proportional relationship to a constant operating range in the LED, it is necessary to increase the driving current in order to acquire a high brightness. However, if the driving current is increased, in proportion to this, power loss in the LED is increased, and most of energy is converted into heat. As a result, the LED is subjected to a high temperature.
Here, because if the temperature is lower, the light emitting efficiency or current-light conversion efficiency becomes higher as the characteristic of the LED, therefore, when the LED is subjected to a high temperature, there will be a problem of lower brightness.
Moreover, when the high-temperature LED continues to be operated, the operating life thereof becomes short, and further there is a possible that transparency of the resin for sealing the LED is decreased, because of change in color due to heat. Therefore, there is a critical problem of operating life and reliability when the LED is used for the high brightness and high output.
To solve the above-described problems, the measures for heat release of the light emitting element are indispensable, and therefore several light emitting elements having heat release means have been proposed. As one of them, a light emitting element in which a pair of conductive members made of a metal material having heat conductivity are fixed by an insulative member and an LED is mounted to straddle the pair of conductive members, is proposed (for reference, see Japanese Patent Laid-Open H 11-307820, claims, FIG. 7).
Hereinafter, a conventional light emitting element will be explained referring to FIG. 9.
In FIG. 9, 1 is a conventional surface-mounted type light emitting element, 2a and 2b a pair of conductive members formed by a metal material having heat conductivity, 3 an insulative member for separating electrically and fixing the pair of conductive members 2a and 2b. 
3a is a concave portion provided on an upper portion of the insulative member 3 so as to expose a portion of the conductive members 2a and 2b. is an LED chip which is mounted to straddle the pair of conductive members 2a and 2b exposed by the concave portion 3a of the insulative member 3, and is electrically connected and thermally coupled with the conductive members 2a and 2b. 5 is a transparent sealing member for sealing the LED chip 4. 6 is a printed-circuit board having conductive patterns 6a and 6b on which the light emitting element 1 is mounted.
Here, when a driving current is applied from the conductive patterns 6a and 6b to the LED chip 4, while the LED chip 4 emits light, simultaneously, the LED chip generates heat as a result of occurring power loss. The heat is transmitted efficiently to the conductive members 2a and 2b coupled thermally with the LED chip 4 and therefore if the printed-circuit board 6 is made of a material of excellent heat conductivity, it is possible to accomplish a good heat radiation effect.
There has been proposed another light emitting element having heat release means, in which a base for mounting an LED chip and lead frames as terminal electrodes are the same material together, the lowest surfaces of the base and lead frames are positioned at the generally same level and the base is mounted directly on the printed-circuit board (for reference, see Japanese Patent Laid-Open 2002-252373, Claims, FIG. 3).
Hereinafter, still another conventional light emitting element will be explained referring to FIG. 10. In FIG. 10, 10 is a conventional light emitting element, 11 a base, 12a and 12b lead frames. The base 11 and the lead frames 12a, 12b are made of the same material, the lowest surfaces of the base 11 and the lead frames 12a, 12b are positioned in the generally same level. 13 is an LED chip which is mounted on a bottom portion of the base 11 and coupled thermally with the base 11.
14a and 14b are wires comprising metal thin wires, which connect electrically an anode and a cathode of the LED chip 13 with the lead frames 12a and 12b. 15 is a transparent sealing member for sealing the base 11, the lead frames 12a, 12b, and the wires 14a, 14b together with the LED chip 13. 16 is a printed-circuit board on which the lead frames 12a, 12b are mounted through soldering 17. Here, because the lowest surface of the base 11 is positioned in the same level as that of the lead frames 12a, 12b, the base 11 is also fitted closely with the printed-circuit board 16 and fixed through the soldering 17, and the base 11 and the printed-circuit board 16 are thermally coupled.
Here, when a driving current is applied through the printed-circuit board 16 to the LED chip 13, the LED chip 13 which emits light simultaneously becomes high temperature by power loss. The heat is transmitted efficiently to the base 11 coupled thermally with the LED chip 13 and further because the base 11 is coupled thermally with the printed-circuit board 16, if the printed-circuit board 16 is made of a material of good heat conductivity, good heat release effect can be achieved. There has been made a proposal in which through-holes (not shown) by the conductive patterns are provided on the printed-circuit board 16 and a heat release member (not shown) is disposed on a back surface of the printed-circuit board 16 to transmit heat through the through-holes to the heat release member.
However, although good heat release effect can be expected to the light emitting element 1 shown in FIG. 9, if a metal core substrate or the like of good heat conductivity is used for the printed-circuit board 6, good heat release effect cannot be expected to a printed-circuit board of glass epoxy material or the like which is inexpensive.
In other words, heat conductivity of the glass epoxy material or the like is small to be a degree of few hundredth part of a copper alloy which is a material of the metal core substrate and therefore heat is not transmitted because of a large thermal resistance of the glass epoxy material or the like. Consequently, the utilization of the metal core substrate is indispensable in order to achieve good heat release effect, but the light emitting device is expensive because the material of the substrate is limited, and it is difficult for the metal core substrate to provide wirings in both surfaces thereof, and a high-density mounting is also difficult.
Furthermore, because the metal core substrate is made of a conductive material, the substrate must be insulated by provision of an insulative layer on a surface of the substrate, but because heat conductivity of the insulative layer is insufficient, sufficient heat release effect cannot be expected to the insulative layer. Because the insulative member 3 is formed by a material of low reflectivity, a force for collecting light by the concave portion is weak and therefore it is not possible to expect large increment of light emitting efficiency.
Moreover, in the light emitting element shown in FIG. 10, there is a problem similar to this. That is to say, because the base 11 is mounted in close relation with the printed-circuit board 16, heat-transmission from the base 11 to the printed-circuit board 16 is very effective, but because the printed-circuit board 16 is made of the glass epoxy material or the like, the heat conductivity is not effective, good heat release effect cannot be expected, further the metal core printed-circuit substrate is required. Moreover, because the structure, in which the heat release member is disposed on the back surface of the printed-circuit board 16, is formed by disposing the printed-circuit board 16 between the base 11 and the heat release member, the base 11 is not coupled thermally with the heat release member, and a structure of the trough-holes in the printed-circuit board 16 is also not effective to enhance heat connection between the base and the heat release member, and therefore significant improvement in heat release effect is not expected.